1. Field of the Invention
The invention is directed to a process for preheating scrap in which gaseous media are heated by the sensible heat of flue gas which is after-burned in a combustion chamber and cleaned in a filter. The invention is further directed to a device for carrying out this process.
2. Description of the Prior Art
In steel production, attempts are generally made to exploit the energy generated in the production process to the fullest extent possible. This includes the use of sensible heat from scrap melting furnaces for preheating burden, in particular scrap.
The scrap used in steel works, depending on provenance, contains varying amounts of organic components, e.g. coats of paint, adhering oil, remainders of textile, plastic or cable, particularly in scrap from automobiles or similar sources, as well as paper, plastic and wood which end up in the scrap in the course of collection and turnover.
In a steel production installation known from DE 30 38 515 A1, thermal energy of the waste gases of an electric furnace is used for preheating materials such as scrap, iron and iron alloys. In so doing, the flue gas exiting the furnace arrives in a combustion chamber and is then fed directly to the burden located in the charging bucket via a bypass line or a transmission line. A disadvantage of this process is that unburned or partially burned noxious matter and toxic components escape from the melting furnace and from the scrap preheater connected in series with the furnace.
A process and a device for preheating materials by hot flue gases is known from EP 0 078 446 A1, wherein contact between the preheating material and the flue gas is prevented. It is suggested herein to connect the flue gas line with a heat exchanger and to connect the secondary side of the heat exchanger with a preheating oven via a feedline and a return line. Further, in order to increase rapid circulation of the heat exchanging medium, a circulating unit is suggested for accelerating the movement of the medium, i.e. air or protective gas, circulating through the system. A disadvantage in this known process is a progressively increasing charging of the recirculated air with noxious matter from the burden heated in the preheating station.
Furthermore, in a process and an installation for preheating scrap to be introduced into an electric furnace known from DE 35 21 569, the preheating gas is guided through a scrap basket and a partial flow of the preheating gas based on the charging of the preheating gas in the preheating circuit with noxious substances and water vapor is mixed in with the furnace gases in the hot region of these furnace gases where the noxious substances burn. The amount of preheating gas that corresponds to the partial flow is replaced by fresh preheating gas. In a preferred arrangement, air is used as the preheating gas and the air guided in the circuit is charged with organic and inorganic substances which are absorbed from the scrap and which carbonize already at relatively low temperatures.
The above-mentioned processes known from operating practice for preheating scrap in electric steel works by means of the hot dust-laden waste gases from the electric arc furnace have the following disadvantage in particular. The waste gases contain oxygen in varying proportions. In the preheating of scrap, the gases cool off and exit the scrap preheating device as heating commences at a low temperature which increases in the course of the heating process. The combustible components are evaporated or carbonized until reaching the ignition temperature for the contained combustible components, since the oxygen content of the waste gas is not sufficient for combustion. In so doing, unburned carbonizing gases and organic pollutants such as dioxin and furan are formed. In order to prevent environmental loading, the waste gas released from the scrap is first heated by adding sufficient amounts of oxygen until the unburned components are reliably burned as well. Often, a portion of the waste gas from the smelting unit is used for this purpose, so that its energy is not available for preheating scrap. Accordingly, the occurring dioxins and furans are not destroyed and must be removed by means of activated coal or catalysts.
The combustion residues (fly ashes) combine with and contaminate the metallurgic dusts occurring in the melting vessel when melting scrap. Owing to the formation of dioxin and furan when preheating scrap, the dusts collected in the filter are also contaminated. In countries having stricter environmental legislation, the disadvantages resulting from the conventional preheating of scrap described above have prevented such preheating of scrap, although it is highly desirable for energy-related reasons.